1. Field of the Invention
This invention relates to a multilayer film structure for soft X-ray optical elements, and more particularly to a multilayer film structure for soft X-ray optical elements in which a high reflectivity can be attained.
2. Description of the Related Art
In soft X-ray field, research and development have been actively carried out recently with respect to reflecting mirrors, reflection type filters, or soft X-ray optical elements such as polarizers, transmission type elements and the like wherein ultra-thin films of two materials each having a thickness of about several nm and a suitable optical constant are alternately laminated to form a multilayer film structure, whereby constructive interference is realized with respect to soft X-ray.
Such soft X-ray optical elements are utilized in development of X-ray microscopes, X-ray telescopes, X-ray laser equipments, or the like.
Meanwhile, in a soft X-ray optical element having a multilayer film structure, reflectivity 58% (angle of incidence .theta.=3.degree. ) has been obtained at 13.8 nm wavelength by means of a Mo/Si multilayer film which is produced by laminating fifty (50) bi-layers of Mo and Si thin films (one bi-layer is formed from one set of a Mo thin film and an Si thin film, and the same shall apply hereinafter).
There has been, however, such a problem that no high reflectivity is obtained at a wavelength of 10 nm or less, for example, in a so-called "water window" zone (a wavelength of from 2.332 nm to 4.368 nm) being a wavelength region wherein C is absorbed, while O is not absorbed, shown in FIG. 1.